This invention relates to an oxygen top-blowing steel making process wherein a powder of flux is supplied to promote slag formation and at the same time, a gas is blown from below the molten steel to increase the force of stirring the molten steel and slag.
The oxygen top-blowing steelmaking process is widely used in Japan, and it blows oxygen through a lance onto a converter charge comprising molten iron, scrap and auxiliary materials. The auxiliary materials comprise quick lime, limestone, dolomite and iron ore, and if they are charged into the furnace as powder, they are thrown off due to carbon monoxide gas generated by the reactions that occur in the furnace, and to prevent this, they are charged as a mass.
However, since quick lime and limestone mainly consist of CaO which melts at as high as about 2570.degree. C., it is difficult to achieve the desired slag formation by allowing them to dissolve completely within the blowing period. In other words, it is difficult to form a highly reactive slag and perform effective dephosphorization and desulfurization as well.
To solve this problem, the LD-AC process (or OLP process) has been developed. This process refines molten iron by blowing an oxygen jet onto the surface of the bath together with a quick lime powder serving as a flux. The process achieves satisfactory dephosphorization and desulfurization because the lime powder suspended in the oxygen jet is directly supplied to the fire point and rapidly forms a reactive slag. But the powder suspended in the oxygen jet causes a pressure drop in the jet, and in addition to that, the powder wears and damages the Lavel-type nozzle to reduce the rate of the oxygen jet supplied. The supply of such low-rate oxygen blow causes a great amount of FeO to be formed and this results in frequent slopping that makes the operation difficult to control. What is more, the LD-AC process requires an apparatus for bringing the powder into the high-pressure oxygen jet and this adds to the cost of initial investment. These reasons plus low efficiency in steelmaking explain why the LD-AC process has yet to be used in commercial steelmaking operation in Japan.
On the other hand, it is known that in the oxygen top-blowing steelmaking process, the stirring of the molten steel becomes weak in the last stage of blowing as a result of reduction in the decarburization rate. Therefore, to promote the slag formation and the reaction between the molten steel and slag, the combined blowing process has been proposed wherein the loss in the stirring of the molten steel is made up for by a gas such as argon or nitrogen which is blown from below the furnace while oxygen is being blown onto the surface of the molten steel.
To dissolve the flux within the period of blowing, FeO is necessary for lowering the melting point of CaO which is the primary component of the flux, but the combined blowing process produces a smaller amount of FeO perhaps due to the increased stirring force. Therefore, in this process, slag formation does not proceed satisfactorily unless some degree of decarburization has taken place.
If these defects of the oxygen top-blowing steelmaking process are eliminated, the advantages of the process, i.e. high efficiency in steelmaking, high-quality product containing minimum impurity elements, and low cost, can be enjoyed to the fullest extent.